Temperature control for electrically heatable window

ABSTRACT

A laminated electrically heatable window has a transparent electrically conductive layer embedded in the window to act as a sheet resistor, and a conductive bus bar in contact with the resistive layer. A portion of the bus bars extends outside an edge of the window for making electrical contact with a power supply such as an automobile alternator. The temperature sensor for preventing overheating of the windshield is mounted in thermal contact with at least one of the bus bars external to the edge of the window rather than in thermal contact with the window itself.

United States Patent 1 1 Levin Feb. 5, 1974 [54] TEMPERATURE CONTROL FOR 2,490,433 12/1949 Gunning et a1. 219 522 ELECTRICALLY HEATABLE WINDOW 2,507,036 5/1950 McCrumm et a1 219/203 Berton P. Levin, Santa Monica, Calif.

Assignee: The Sierracin Corporation, Sylmar,

Canada Filed: Oct. 13, 1972 Appl. No.: 300,403

Inventor:

US. Cl. 219/203, 219/522 Int. Cl B60] 1/02 Field of Search,... 219/203, 522, 543; 244/121,

244/134 A, 134 D, 134 R; 340/234 References Cited UNITED STATES PATENTS 2/1949 Mershon 219/203 X 6/1952 Mershon 219/203 X Primary Examiner-C. L. Albritton Attorney, Agent, or Firm-Christie, Parker & Hale ABSTRACT A laminated electrically heatable window has a transparent electrically conductive layer embedded in the window to act as a sheet resistor, and a conductive bus bar in contact with the resistive layer. A portion of the bus bars extends outside an edge of the window for making electrical contact with a power supply such as an automobile alternator. The temperature sensor for preventing overheating of the windshield is mounted in thermal contact with at least one of the bus bars external to the edge of the window rather than in thermal contact with the window itself.

15 Claims, 5 Drawing Figures TEMPERATURE CONTROL FOR ELECTRICALLY HEATABLE WINDOW BACKGROUND It has become highly desirable to provide electrical heating in windows of automobiles for removing accumulations of ice and snow and inhibiting condensation of fog and frost. One type of such electrically heatable window for automobiles has a very thin conductive metal film laminated within the window and extending over most of the area of the window. The metal film is sufficiently thin that it is transparent and when a current is passed between its side edges it serves as a sheet resistor. Since the resistive heater layer extends over most of the area of the window, much more uniform heating is obtained than with discrete wires or other conductors embedded in the window.

Typically, such an electrical heatable window has the thin metalfilm vacuum deposited on a carrier film of transparent plastic, such as polyethylene terephthalate. The carrier film is sandwiched between a pair of polyvinyl butyral interlayers which are in turn sandwiched in between the face sheets of glass. Electrically conductive bus bars, typically of thin copper foil, are laminated into the heatable window between the carrier film and interlayers so as to be in electrical contact with the thin conductive film. Electric current applied between busbars along opposite edges of conductive areas within the window assures uniform heating over the area of the window.

The ends of the conductive bus bars are carried beyond an edge of the window for making electrical connection to the automobile alternator for applying power to the window. In a typical arrangement the ends of the foil bus bars beyond the edge of the window are substantially completely encased in plastic except for an area to which electrical contact to the balance of the car wiring is made. When the electrically heatable window is activated for removing or inhibiting fog or ice, power is applied while the window is at low temperature. The temperature of the window is typically permitted to rise a substantial amount above the freezing point of water to assure complete fog and ice removal. It is necessary, however, to place an upper limit on the temperature that can be reached by the window. Thus, for example, if one should activate the electrically heatable window on a hot day, the temperature might become sufficiently high that damage to the plastic interlayers or cracking of the glass can occur. For this reason, temperature limiting switches have been fastened directly to electrically heatable windows for cutting off power and limiting the maximum temperature that the window can reach when heated. The temperature sensors are typically inexpensive, thermally responsive switches electrically connected to the field control windings of a high power level automobile alternator.

The practice has been to connect a thermally responsive switch to the window by adhesively bonding it to the glass. This has raised substantial problems in handling the window in manufacturing operations and in making electrical contact to the leads from the sensor. The protruding rigid switch mounted on the glass can result in breakage of the glass during handling of the window. The electrical leads are difficult to route for connection to the alternator.

It is therefore desirable to provide an arrangement for limiting the temperature of an electrically heatable window without the hazard of mechanical damage thereto. Convenience in electrical connection is also desirable.

BRIEF SUMMARY OF THE INVENTION There is, therefore, provided in practice of this invention according to a presently preferred embodiment, a laminated electrically heatable window having a transparent electric resistive layer embedded therein with conductive bus bars in contact with the resistive layer and a portion of the bus bars extending beyond an edge of the window, and an improved arrangement for limiting temperature of the window comprising a temperature responsive sensor in thermal contact with a portion of at least one of the bus bars external to the edge of the window wherein the thermal characteristics of the bus bar-sensor. combination are correlated to the thermal characteristics of the window.

DRAWINGS These and other features and advantages of the invention will be appreciated as the same becomes better understood by reference to the following detailed description of a presently preferred embodiment when considered in connection with the accompanying drawings wherein:

FIG. 1 illustrates in face view a typical electrically heatable windshield constructed according to princi- DESCRIPTION FIG. 1 illustrates in face view a typical electrically heatable automobile windshield constructed according to principles of this inventionpAs illustrated in this presently preferred embodiment, the windshield is transparent throughout most of its extent and includes an electrically conductive or resistive layer of vacuumdeposited metal for electrical heating. The conductive layer of metal, which is not specifically illustrated herein, is sufficiently thin to be transparent.

Along the upper edge of the windshield there is a copper foil bus bar 10 such as for example one made according to teachings of U.S. Pat. No. 3,612,745. A lead 11 similar to the bus bar is brought down along one side edge of the window and extends beyond the bottom edge of the window in a tab 12. The tab is seen in greater detail in FIGS. 2 and 3. A second bus bar 13 extends along the bottom edge of the window and terminates in a second lead 14 in the tab 12. A third bus bar 16 extends approximately half-way across the window along the lower edge and between the first and second bus bars. The third bus bar has a lead 17 extending beyond the bottom edge of the window into the tab 12.

An electrical isolation line 18 extends vertically across the window and divides the conductive film into two conductive areas 19 and 20. The electrical isolation line is merely an extremely fine scribe line that interrupts the electrically conductive film in the window and typically this line is almost invisible. Additional isolation lines (not shown) may be provided between adja-' cent bus bars and along leads for limiting current flow therebetween.

During operation of the electrically heatable window, current is passed between the first bus bar and the second bus bar 16 for heating the resistive area 19. Similarly, current is passed between the top bus bar 10 and the portion of the lower bus bar 13 beyond the end of the other lower bus bar 16 for electrically heating the second resistive area 20 of the windshield. In a typical embodiment the current through the two conductive areas 19 and 20 are two phases of a three-phase power supply. The third phase is typically applied to the back window of the automobile. Because of this arrangement, the current flow through the lead 11 is greater than the current flow through either of the leads 14 or 17 to the bus bars along the lower edge of the window by a factor of 3:

FIGS. 2 and 3 illustrate the tab 12 where electrical connection is made to the bus bars within the windows in face view and transverse cross section respectively. The windshield has two face plies 22 of glass. A pair of polyvinyl butyral interlayers 23 are bonded to and between the sheets of glass. A carrier film 24 of polyethylene terephthalate is sandwiched between the interlayers 24 over most of the area of the window and is bonded to both interlayers.

In the region illustrated in the cross section of FIG. 3 one of the bus bar leads 1 l is seen between the carrier film and one of the interlayers. Within the window a short distance from the bottom edge adjacent the tab 12, the plastic carrier film 24 ends and the bus bar lead 11 makes a slight jog. It will be recognized of course that FIG.3 is greatly exaggerated and this jog is no more than a few thousanths of an inch. A pair of flexible plastic sheets 26 are positioned on the opposite faces of the bus bars 11. These sheets 26 commence at about the point carrier film 24 ends and extend beyond the edge of the window for forming the tab 12. The sheets 26 have an aggregate thickness about the same as the carrier film and are firmly attached to the window by the edges laminated between the interlayers. Throughout much of their extent the plastic sheets 26 are bonded to each other and in some areas they are bonded to the bus bar leads 11, 14, and 17 sandwiched therebetween. About the only requirement for the plastic sheets 26 is that they have sufficient strength to resist up to a pound pull during handling and resist damage up to the maximum temperature to be encountered by the window. One or both of the plastic sheets 26 has apertures, (not shown), adjacent the faces of the bus bar leads ll, 14, and 17 so that electrical contact can be made thereto for heating the window. Such electrical contact is indicated schematically in FIG. 2 by connections to the alternator 27 of the automobile. This much of an electrically heatable automobile windshield, power supply, and electrical connections therebetween is conventional.

Previously, it has been the practice to apply a bimetal thermally responsive switch to the windshield within one of the conductive areas 19 or so that the temperature of the windshield is directly monitored and the field control current of the automobile alternator cut off when a predetermined temperature is reached in the windshield. In practice of this invention, however, the temperature of the window is not directly sensed in order to avoid the problems associated with the attachment of sensor on the glass.

Instead, in practice of this invention, a conventional temperature responsive switch 28 is adhesively bonded on the flexible tab 12 in a region thereof that overlies a portion of one of the electrical bus bar leads. It is found that correlation can readily be made between the thermal characteristics of the bus bar and the thermal characteristics of the electrically heatable windshield. This is the case since the tab on which the thermally responsive switch is mounted is typically in a thermal environment sufficiently isolated from the passenger and engine compartments of the automobile that its temperature reasonably approximates the temperature of the windshield itself. The thermally responsive switch 28 is therefore mounted in thermal contact with one or more of the bus bar leads. The leads 29 from the temperature switch are connected directly to the alternator field excitation control 31 in a conventional manner.

The power density dissipated in the bus bars, that is, electrical energy converted to heat, is easily related to the power dissipation in the windshield itself. Assuming a constant distance of L inches between the bus bars, the windshield power density is given by P= e /pL where P is the windshield power density in watts per square inch, e is the rms voltage of the alternator, and p is the sheet resistivity of the thin conductive film in the windshield in ohms'per square. Even though the two conductive areas 19 and 20 may not have the same width the power density is the same in both. This is not true of the current to the two conductive areas which is given by i= ew/pL where i is the current in amperes, and w is the effective width of the windshield segment in inches.

The power density in the bus bar can be expressed in terms of the current flowing through the bus bar. The power density is given by the relation PB 11 P iz/ n where all symbols are analogous to those for the windshield and the subscript B refers to the bus bars. The ratio of the power densities in the bus bar and wind shield is given by the relation PB/P= 13 PP B/WB2 The current carried by the two lower bus bars 13 and 16 is just the current through the respective conductive areas for the windshield while the current carried by the upper bus bar 10 is \[3 times this value since it is common to two phases of the alternator output. Thus for the bus bar 10 common to both segments of the windshield i 31* Substituting this and equation (2) into equation (4) yields the relation for the maximum power density in the bus bar relative to the power density in the window according to the following relation Taking the limiting specified values for a typical electrically heatable windshield for an automobile, it is found that the maximum power dissipation in the upper common bus bar is no more than 86 percent of the power dissipation in the heated area of the window. The power dissipation in the lower bus bars 13 and 16 is commensurately lower because of the smaller current flowing therethrough. The power dissipation in the leads in the tab outside the window is similar to the power dissipation of the bus bars within the window since they are merely continuations thereof.

Since the power dissipation within the bus bar lead can be determined with some degree or precision relative to the power dissipation within the conductive area of the windshield, suitable adjustments can be made in the temperature at which the thermally responsive switch actuates so that a desired maximum temperature is not exceeded by the window.

It will be noted from equation (5) that the power dissipation in the bus bar is directly proportional to its resistivity in ohms per square and inversely proportional to the square of its width. One can therefore readily alter the dimensions of the electrical bus bar lead within the tab 12 to provide a local power dissipation rather closely matched to the power dissipation of the window itself. FIG. 2 illustrates such an arrangement wherein the bus bar lead 17 has a short portion 32 which is narrower than the balance of the bus bar in thermal contact with the thermal switch 28. Since the width of the bus bar lead is reduced, the power dissipation is increased and if desired the ratio P /P can be made equal to 1. That is, the power dissipation in a portion of the bus bar can be made the same as the power dissipation within the window. In that situation and with comparable thermal environment, the temperature in this portion of the bus bar lead, as sensed by the thermal switch, is substantially the same as the temperature in the window. It will be apparent, of course, that the power dissipation of the portion of the bus bar in thermal contact with the temperature switch can also be predetermined by reducing its thickness; however, narrowing the bus bar is substantially easier to effect and control.

It may be found that reducing the thickness or width of the bus bar adjacent the sensor is undesirable if the mechanical properties are impaired to the point that breakage can occur. Thus, if the bus bar is made too narrow in an effort to match its power characteristics to those of the window, it may be too fragile to withstand practical assembly line manufacturing operations. As noted in equation (3), the power is proportional to the resistivity as well as being width sensitive. In another embodiment, it has been found convenient to remove a short segment of the copper bus bar and substitute a high resistivity material in the region in thermal contact with the sensor. The window heating current flows through the insert in series with the balance of the bus bar. For example, a gap one half inch or so of the copper bus bar is deleted and a short piece of Nichrome, Chromel, lnconel or other high resistivity metal is soldered in place in the portion of the bus bar in the tab. The dimensions and resistivity of the insert are selected to approximate the thermal characteristics of the window. The sensor is then mounted over the high resistivity insert.

It will be apparent that if desired, the known relation between the power dissipation in the bus bar and the power dissipation in the window can be employed for selecting the temperature at which the switch activates rather than altering the power dissipation characteristics of a portion of the bus bar. Thus, for example, the temperature responsive switch can be set to open at a bus bar temperature that is in fact lower than the temperature of the window but bearing a predetermined relation thereto. If desired, the switch can be mounted on the connector that makes electrical contact with the bus bars rather than directly on the flexible tab. In either case, an important feature is that the temperature responsive switch is in effective thermal contact with a portion of the bus bar external to the window ratherthan being mounted on the window itself.

In some embodiments the inherent thermal environment of the tab on the window may differ from that of the windshield itself. Thus, for example, the electrical leads connected to the tab may have sufficient heat conductivity to be a partial heat sink so that the temperature of the tab differs from that of the windshield. FIGS. 4 and 5 illustrate schematically arrangements for minimizing the thermal environment differences between the tab and windshield. In the arrangement illustrated in FIG. 4 the portion of the tab 12 opposite from the switch 28 is brought into thermal contact with a heat sink 33. In the arrangement illustrated in FIG. 5 the side of the thermal switch 28 opposite from the tab 12 is brought into thermal contact with a heat sink 34. The heat sink can be a structural member adjacent the windshield or an added heat path. These heat sinks can have any desired thermal diffusivity sufficient for maintaining the environment of the temperature responsive switch 28, approximately equivalent to the thermal environment of the windshield itself. In lieu of a heat sink that extracts large amounts of heat from the sensor-bus bar combination, the sink may be an insulator of fairly low thermal diffusivity so that the combination can rise in temperature above the surrounding structure. Whether of high or low thermal diffusivity, the heat sink typically conveys heat away from the combination when power is applied to the window. In this way, the temperature response of the switch provides an accurate reflection of the temperature of the windshield even though it is out of thermal contact therewith.

Although limited embodiments of temperature sensor in combination-with an electrically heatable windshield have been described and illustrated in detail herein, many modifications and variations will be apparent to one skilled in the art. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

Since the switch is on the tab the risk of damaging the windshield due to handling is substantially reduced. In addition, the lead wires from the switch can be routed with the other electrical leads with much greater facility.

What is claimed is:

1. In a laminated electrically heatable window having a transparent electrically resistive layer embedded in the window, conductive bus bars embedded in the window in contact with the resistive layer and a portion of the bus bars extending outside an edge of the window, an improved combination for limiting temperature of the window comprising:

a temperature responsive switch in thermal contact with a portion of at least one of the bus bars external to the edge of the window; and

means for approximately correlating the thermal characteristics of the bus bar-sensor combination to the thermal characteristics of the window.

2. In a combination as defined in claim 1 the improvement wherein the means for correlating com prises a portion of the bus bar external to the window having a width narrower than another width of the bus bar; and wherein the temperature responsive switch is in thermal contact with the narrowed portion of bus bar.

3. In a combination as defined in claim 1 the improvement wherein the means for correlating comprises a heat sink in thermal contact with the bus barsensor combination.

4. In a combination as defined in claim 1, the improvement wherein the means for correlating comprises a portion of the bus bar external to the window having an electrical resistivity higher than the resistivity of another portion of the bus bar in series therewith; and wherein the temperature responsive switch is in thermal contact with the higher resistivity portion.

5. In a combination as defined in claim 1, the improvement wherein the means for correlating comprises a portion of bus bar external to the window and in thermal contact with the switch, said portion having a power dissipation similar to the power dissipation of the resistive layer.

6. In a laminated electrically heatable window having an electric resistive element embedded in the window and an electric lead extending out of an edge of the window, an improved combination for limiting temperature of the window comprising:

a selected region of the electric lead external to the window with a power dissipation characteristic bearing a known relation to a power dissipation characteristic of the resistive element; and

a temperature responsive sensor in sufficient thermal contact with the selected region of the electric lead to be thermally responsive to the selected region.

7. In a laminated electrically heatable window having an electric resistive element embedded in the window and an electric lead extending out of an edge of the window, an improved combination for limiting temperature of the window comprising:

a selected region of the electric lead external to the window having a constricted cross section as compared with the cross section of another portion of the electric lead, and with a power dissipation characteristic bearing a known relation to a power dissipation characteristic of the resistive element; and

a temperature responsive sensor in thermal contact with the selected region of the electric lead.

8. In a combination as defined in claim 6, the improvement wherein the selected region of the electric lead is in thermal contact with a heat sink.

9. In a laminated electrically heatable window having an electric resistive element embedded in the window and an electric lead extending out of an edge of the window, an improved combination for limiting temperature of the window comprising:

a selected region of the electric lead external to the window having an electrical resistivity higher than the resistivity of another portion of the electrical lead, and with a power dissipation characteristic bearing a known relation to a power dissipation characteristic of the resistive element; and

a temperature responsive sensor in thermal contact with the selected region of the electric lead.

10. In a combination as defined in claim 5, the improvement wherein the temperature responsive sensor is in thermal contact with a heat sink.

11. In a laminated electrically heatable window having a transparent electrically conductive layer embedded in the window, conductive bus bars embedded in the window in contact with the conductive layer, and a portion of the bus bars extending outside an edge of the window, an improved combination for limiting temperature of the window comprising:

a temperature responsive switch mounted in effective thermal contact with a bus ba'r external to the window and out of effective thermal contact with the conductive layer.

12. In a combination as defined in claim 11, the further improvement wherein the power dissipation of a portion of the bus bar in effective thermal contact with the temperature responsive switch is different from the power dissipation of another portion of the bus bar.

13. In a combination as defined in claim 12, the improvement wherein the width of a bus bar in effective thermal contact with the switch is narrower than the width of another portion of the bus bar.

14. In a laminated electrically heatable window as defined in claim 10, the improvement wherein the electrical resistivity of a bus bar in effective thermal contact with the switch is higher than the resistivity of another portion of the bus bar.

15. In a laminated electrically heatable window as defined in claim ll,the improvement wherein the portion of bus bars external to the window comprises a flexible tab on an edge of the window and wherein the switch is mounted on the tab. 

1. In a laminated electrically heatable window having a transparent electrically resistive layer embedded in the window, conductive bus bars embedded in the window in contact with the resistive layer and a portion of the bus bars extending outside an edge of the window, an improved combination for limiting temperature of the window comprising: a temperature responsive switch in thermal contact with a portion of at least one of the bus bars external to the edge of the window; and means for approximately correlating the thermal characteristics of the bus bar-sensor combination to the thermal characteristics of the window.
 2. In a combination as defined in claim 1 the improvement wherein the means for correlating comprises a portion of the bus bar external to the window having a width narrower than another width of the bus bar; and wherein the temperature responsive switch is in thermal contact with the narrowed portion of bus bar.
 3. In a combination as defined in claim 1 the improvement wherein the means for correlating comprises a heat sink in thermal contact with the bus bar-sensor combination.
 4. In a combination as defined in claim 1, the improvement wherein the means for correlating comprises a portion of the bus bar external to the window having an electrical resistivity higher than the resistivity of another portion of the bus bar in series therewith; and wherein the temperature responsive switch is in thermal contact with the higher resistivity portion.
 5. In a combination as defined in claim 1, the improvement wherein the means for correlating comprises a portion of bus bar external to the window and in thermal contact with the switch, said portion having a power dissipation similar to the power dissipation of the resistive layer.
 6. In a laminated electrically heatable window having an electric resistive element embedded in the window and an electric lead extending out of an edge of the window, an improved combination for limiting temperature of the window comprising: a selected region of the electric lead external to the window with a power dissipation characteristic bearing a known relation to a power dissipation characteristic of the resistive element; and a temperature responsive sensor in sufficient thermal contact with the selected region of the electric lead to be thermally responsive to the selected region.
 7. In a laminated electrically heatable window having an electric resistive element embedded in the window and an electric lead extending out of an edge of the window, an improved combination for limiting temperature of the window comprising: a selected region of the electric lead external to the window having a constricted cross section as compared with the cross section of another portion of the electric lead, and with a power dissipation characteristic bearing a known relation to a power dissipation characteristic of the resistive element; and a temperature responsive sensor in thermal contact with the selected region of the electric lead.
 8. In a combination as defined in claim 6, the imProvement wherein the selected region of the electric lead is in thermal contact with a heat sink.
 9. In a laminated electrically heatable window having an electric resistive element embedded in the window and an electric lead extending out of an edge of the window, an improved combination for limiting temperature of the window comprising: a selected region of the electric lead external to the window having an electrical resistivity higher than the resistivity of another portion of the electrical lead, and with a power dissipation characteristic bearing a known relation to a power dissipation characteristic of the resistive element; and a temperature responsive sensor in thermal contact with the selected region of the electric lead.
 10. In a combination as defined in claim 5, the improvement wherein the temperature responsive sensor is in thermal contact with a heat sink.
 11. In a laminated electrically heatable window having a transparent electrically conductive layer embedded in the window, conductive bus bars embedded in the window in contact with the conductive layer, and a portion of the bus bars extending outside an edge of the window, an improved combination for limiting temperature of the window comprising: a temperature responsive switch mounted in effective thermal contact with a bus bar external to the window and out of effective thermal contact with the conductive layer.
 12. In a combination as defined in claim 11, the further improvement wherein the power dissipation of a portion of the bus bar in effective thermal contact with the temperature responsive switch is different from the power dissipation of another portion of the bus bar.
 13. In a combination as defined in claim 12, the improvement wherein the width of a bus bar in effective thermal contact with the switch is narrower than the width of another portion of the bus bar.
 14. In a laminated electrically heatable window as defined in claim 10, the improvement wherein the electrical resistivity of a bus bar in effective thermal contact with the switch is higher than the resistivity of another portion of the bus bar.
 15. In a laminated electrically heatable window as defined in claim 11,the improvement wherein the portion of bus bars external to the window comprises a flexible tab on an edge of the window and wherein the switch is mounted on the tab. 